Computational Design, Synthesis, and Structure Property Evaluation

Article Cite This: J. Org. Chem. 2018, 83, 3453−3466

pubs.acs.org/joc

Computational Design, Synthesis, and Structure Property Evaluation of 1,3-Thiazole-Based Color-Tunable Multi-heterocyclic Small Organic Fluorophores as Multifunctional Molecular Materials Rakesh Radhakrishnan and K. G. Sreejalekshmi* Department of Chemistry, Indian Institute of Space Science and Technology, Valiamala Post, Thiruvananthapuram 695 547, India S Supporting Information *

ABSTRACT: Probing the chemical space of luminescent organic materials built on novel cores is highly imperative for its potential to expand the horizons of advanced functional materials. Small organic fluorophores possessing therapeutic traits can contribute to theranostics. We coupled computational and classical synthetic chemistry strategies for the rational design of 5-(hetero-2-yl)-1,3-thiazoles as color-tunable fluorophore core. With the aid of DFT and TD-DFT, we prove that the multi-heterocyclic system is built on a thiazole− het core with three inherent tunable sites on thiazole (C2, C4, and C5) capable of accommodating a panoply of substituents as a multifunctional molecular materials’ platform. This de novo design offered unprecedented freedom to control strength and direction of charge transfer by varying donor−acceptor fragments. A 30-member fluorophore library built on thiazole-thiophene/ furan core was accomplished using commercial reagents by a simple [4 + 1] synthesis. Structure−photophysical property studies revealed large Stokes shift, positive solvatochromism, acidochromism, and color tunability in different solvents and were rationalized using computational calculations. In vitro studies indicated 1a to be active against HL-60 cell lines, suggesting the possibility of expanding the core for theranostics. The lower values of computed hole reorganization energies indicated their potential as hole transporting materials in optoelectronics and widen the scope of these molecules as advanced functional materials.

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INTRODUCTION Heterocycles, the privileged medicinal scaf folds,1 have recently found extended use in diverse fields of chemistry,2−5 materials science,6−12 and biology13−15 by virtue of their interesting and diverse features. In the context of growing attention toward the development of single molecule based trackable therapeutic agents,16,17 it is desirable to explore heterocycles and their amalgamations accomplished through molecular hybridizations18,19 as possible candidates for theranostics and analogous applications. In the theranostic realm, development of minimal architecture fluorophores with drug likeness is highly imperative, concurrently challenging, and yet less investigated, except for a few reports.16,20,21 Appreciative of the difficulties and excitements in novel small organic fluorophore design, we felt it worthy to design classical push−pull chromophores using heterocyclic fragments with diverse properties so that their potential as multifunctional molecules en route to advanced multifunctional materials’22 development can be explored. Among various heterocycles, 1,3-thiazole, a member of the 1,3-azole family with significant pharmaceutical interest and found in numerous drug molecules,23,24 is underexplored for its fluorescence properties.25−27 Our laboratories are expanding the prospects of thiazole chemistry from drug discovery28,29 to the development of multifunctional materials,30,31 typically involving flow of concepts like molecular hybridization and © 2018 American Chemical Society

multi-heterocycle cores from medicinal chemistry to materials chemistry. We expect that such an approach would further broaden the scope of heterocyclic systems in the design and development of multifunctional materials with significant roles in biological scenario,2,32−34 fluorophores for environmental sensing,35 probes for understanding protein folding mechanisms, and for binding site analysis.36−40 Inspired by a diversityoriented fluorescent library concept,41,42 computational-assisted rational design strategies,43 and classical synthetic chemistry methods, we hypothesized multi-heterocyclic fluorophores based on 1,3-thiazole with promising therapeutic properties and attempted the synthesis of de novo fluorescent structures. The dearth of attempts on rational design of thiazole-based fluorophores further warranted detailed understanding of the relationship between molecular structure and photophysical properties to guide the design of better systems with selective (tunable) properties for specialized applications. To this end, we now discuss on the molecular engineering on 1,3-thiazole scaffold for the de novo fluorophore design which started with probing electron densities on the ring at C2, C4, and C5 (Figure 1a) to identify suitable anchoring sites for various heterocycles and functional groups in the development Received: November 24, 2017 Published: January 15, 2018 3453

DOI: 10.1021/acs.joc.7b02978 J. Org. Chem. 2018, 83, 3453−3466

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The Journal of Organic Chemistry

diversity has considerable scope of expanding unexplored regions of chemical space of small molecules for diverse applications.34,52,53 The scarcity of new molecular skeletons with multitude of properties attuned for diverse applications and the challenges in the rational design of tunable cores54,55 further encouraged us to undertake this study. We further discuss on the synthesis and structure property relationship studies on these minimal architecture molecules and evaluate their potential as multifunctional materials.

Figure 1. (a) Mulliken atomic charges on thiazole ring atoms and (b) atomic contribution to HOMO and LUMO electron densities calculated at PBE0 6-31(d,p).

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RESULTS AND DISCUSSION Rational Design of Multi-heterocyclic Core. Our design strategy, primarily targeting a 1,3-thiazole-based fluorophore core development, envisioned the classical approach of coupling donor (D)−acceptor (A) systems to generate push−pull molecules to enable the intramolecular charge transfer (ICT) and subsequently a red shift in the absorption and emission maxima. Heteroaromatics with their potential to act as auxiliary donor and acceptor systems3 prompted us to choose diverse heterocycles around the 1,3-thiazole core, positioned at the electron rich C5, aiming at novel family of band gap controllable systems. Accordingly, nine such heterocycles were chosen and the variations in the energy gaps for the resultant biheteroaryl systems were computed to draw a first hand information on charge delocalization behavior (Figure 2). From the observations, it was rather understood that the charge delocalization behavior on the designed cores could be modulated by introducing strongly electron donating and withdrawing groups at the periphery of biheteroaryl systems to generate diads (D−A), triads (D−A−A), or tetrads (D−A−D− A) with probably interesting photophysical properties. For example, by appending NMe2 at C2 of thiazole and NO2 at C5 of the second heterocycle, the band gap variation from 3.44− 3.75 eV and absorption tunability of 368−405 nm were predicted using gas phase calculation at PBE0/6-31 g(d,p) (Figure 3). Among the several potential fluorophore cores, we chose the thiazole−thiophene core, the one with highest predicted absorption wavelength, for further design modification and optimizations for which various acceptors were introduced at C5 of the thiophene (Figure 4). This resulted in a substantial

of multi-heterocyclic fluorophores. Among the inherent tunable sites, electron density being concentrated on C5 leaving C2 electron poor, as depicted in the Mulliken charge distribution, imparts polarizable character to thiazole, which acts as an auxiliary donor and acceptor.44 Furthermore, on analyzing electron densities in HOMO and LUMO, a significant variation perceived at C4 indicated by lobe size differences (Figure 1b) suggested C4 as a potential tunable site. Since destabilization of HOMO at C4 can be effected by an electron-donating group, we hypothesize that the HOMO−LUMO energy gap will decrease by placing electron-rich substituents at this position. We were also interested in assessing the directionality of charge transfer in the designer systems due to the limited studies on molecular materials with both emission and charge conductions.45 Thus, the key design issues that we attempt to address in this work are (i) selection of appropriate heterocyclic fragments to decorate C2, C4, and C5 of the 1,3-thiazole ring to generate a diversity-oriented yet synthetically viable minimal architecture fluorophore library, (ii) coupling computational chemistry tools with classical approaches in understanding structure−property relationship, and (iii) synthesis and property evaluations to prove our hypothesis of rational design of multifunctional materials through molecular hybridization utilizing a 1,3-thiazole core. It was surprising to note that only a few families of fluorophores such as cyanines,46 BODIPY,47,48 fluorescein,40 coumarin,49,50 and Seoul-Fluor51 were subjected to the study of fundamental understanding of molecular properties, whereas ab initio designs to develop new tunable fluorophore scaffolds coupled with the immense potential of diversity-oriented synthesis (DOS) to generate molecular-

Figure 2. Band gap, absorption wavelength, and force constant predicted in different biheteroaryl cores with percentage atomic contribution to HOMO and LUMO electron densities (in the parentheses). 3454


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Figure 3. Band gap and absorption wavelength tunability in the designed multicore systems.

Figure 4. Plausible variation in absorption wavelength in thienylthiazole core (calculated at PBE0/6-31G(d,p)).

nitrothiophene-2-yl)thiazoles was achieved by the simple reaction of 1-(acyl/aroyl)-3,3-(disubstituted)thioureas in DMF with 2-bromomethyl-5-nitrothiophene in the presence of a base at room temperature for 30−45 min. The products were purified by column chromatography on silica gel and were completely characterized by spectroscopic methods (SI). In addition to the shorter reaction times and good to excellent yield of the products, an explicit feature of the reaction protocol regarding the versatility in accommodating a wide range of reactants also requires special mention in the context of our hypothesis of developing multi-heterocyclic cores. From the reaction scheme, it is understood that the tunable handles at C2 and C4 of the thiazole ring are contributions from the secondary amines and carbonyl chlorides, respectively, which are employed in the thiourea (A) synthesis, whereas the halomethyl reagent (B) utilized in the ring construction step affords the heterocycle in the thiazole−heterocycle (thiazole− het) core. For example, among the library members 1a−v, derivatives 1i, 1j, 1k, 1p, 1q,56 and 1t bear three heterocyclic units, whereas in 1l, four different heterocycles were incorporated, which attests to our success in the construction of multiheterocyclic systems. The synthetic simplicity, versatility, and availability of wide range of starting compounds further encouraged us to expand the library of thiazole−thiophene to other thiazole−het systems. As mentioned previously, by incorporating appropriate heterocycle bearing activemethylene

shift in absorption wavelength from 357−525 nm and band gap difference of 1.25 eV, attesting to the success of our strategy of utilizing C2 → C5 as the direction of ICT in the tunable core design. Further, the force constant value which would reflect on molar extinction coeffficient also varied significantly. In the next step, encouraged by our previous results on the effect of substituents at C456 of thiazole ring on the charge transfer capabilities of the core, we designed several molecules with diverse substituents at thiazole C4 which provided strong evidence of having a second charge-transfer channel. After investigating the band gap and wavelength tunability and hence gathering sufficient clues on the flexibility of the design, we proceeded with the synthesis of the molecules for complementing the computationally predicted properties. Synthesis. When it comes to achieving synthetic procedures to designed molecules, simplicity, efficiency, cost effectiveness, and sustainability of the routes are highly desirable. Cheng et al. reported synthesis of a library of biheteroaryl fluorophores through a metal-catalyzed direct oxidative C−H/C−H cross coupling reactio,54 whereas we successfully identified a [4 + 1] thiazole ring route utilizing carbonyl compounds, secondary amines, and halomethyl heterocycles as building blocks for the modular synthesis of multi-heterocyclic D−A systems in a metal-free reaction protocol56 which is highly favorable from a sustainable chemistry point of view. For example, following the synthetic procedure (Scheme 1) which highly suits the diversity oriented approach, a 22-member library (1a−v) of 5-(53455


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The Journal of Organic Chemistry Scheme 1. General Route for the Synthesis of the Thiazole−Heterocycle Corea

a

Box color represents the number of heterocycles in each system; blue = 4, red = 3, and no color = 2.

turic acid, we synthesized 4 and 5, respectively, in good yield (Scheme 2). In order to unambiguously prove the chemical structures and for understanding the crystal landscapes, we developed single

unit in the thiazole ring formation step, this could be achieved in a straightforward manner as exemplified by the synthesis of novel class of thiazole−furan (TF) systems. Five members of the TF family were synthesized in very good yield by keeping the C4 substituent unvaried and using different amines for C2 diversification. Thus, 2a−e were successfully synthesized following Scheme 1 in very good yield by reacting appropriate thioureas with commercially available 2-(bromomethyl)-5nitrofuran. Continuing with the molecular engineering around the core, and identifying the role of C5 substituents on the het unit in the thiazole−het core, we felt it worth replacing nitrothiophene unit by thiophene carbaldehyde in Scheme 1. Thus, a novel family of 5-(thiazol-5-yl)thiophene-2-carbaldehyde 3 was effectively synthesized. This modification gave a huge impetus to our fluorophore library design since it opened new vistas with the abundance of acceptor fragments, many of them industrially important, that can be utilized for extending πconjugation. Hence, using aldehyde 3, in simple Knoevenagel condensations with malononitrile and 1,3-diethyl-2-thiobarbi-

Scheme 2. Synthesis of π-Extended Thiazoles through Heterocycle C5 Modification in Thiazole−Het Core

3456


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The Journal of Organic Chemistry Table 1. Photophysical Properties of the Selected Compoundsa code

solvent

λabsb/λemc (nm)

Stokes shift (cm−1)

QYd (%)

code

λabsb/ λemc (nm)

Stokes shift (cm−1)

QYd (%)

1d

n-hexane toluene THF DCM ACN DMSO n-hexane toluene THF DCM ACN DMSO n-hexane toluene THF DCM ACN DMSO n-hexane toluene THF DCM ACN DMSO n-hexane toluene THF DCM ACN DMSO n-hexane toluene THF DCM ACN DMSO

446/537 474/580 479/609 492/642 487/646 503/654 445/543 471/585 476/614 487/644 480/640 497/644 433/532 457/574 462/611 470/644 468/641 483/656 439/534 457/577 460/601 470/653 463/673 483/674 447/542 460/577 468/616 476/669 467/671 480/671 554/595 560/628 552/657 566/671 548/688 558/695

3800 3856 4456 4749 4958 5243 4056 4137 4722 5006 5208 4593 4298 4460 5278 5749 5767 5460 4052 4551 5100 5963 6739 5867 3921 4408 5134 6061 6510 5930 1244 1934 2895 2765 3713 3533

12.6 52.5 4.0 0.2 0.05 0.2 10.5 55.5 13.0 1.2 NE NE 3.8 71.5 7.5 0.3 NE NE 23.9 69.1 18.6 1.1 0.3 0.4 12.3 68.3 19.0 0.9 0.1 0.3 4.4 8.8 21.3 22.8 1.9 3.5

1s

433/532 460/560 468/616 476/669 467/671 480/671 433/517 452/556 457/595 466/644 459/654 472/665 425/507 457/550 455/572 469/612 465/630 475/635 401/493 412/511 409/521 415/532 408/537 416/550 497/558 499/584 496/620 506/628 491/644 499/657 470/598 491/671 493/645

4298 3882 5134 6061 6510 5930 3752 4138 5075 5931 6496 6149 3806 3700 4496 4982 5632 5305 4654 4702 5256 5299 5888 5857 2200 2917 4032 3839 4839 4819 4554 5463 4780

17.3 40.8 36.8 12.2 0.7 1.6 87.1 49.7 13.7 1.5 0.1 0.2 5.3 2.3 1.0 0.4 0.4 0.2 3.8 8.8 3.3 6.9 8.4 22.5 2.6 4.0 8.0 10.7 6.0 7.4 19.2 3.0 0.1

1g

1i

1m

1r

5

1v

2a

3

4

1u

Photophysical properties measured in solutions of concentration 10−5 mol L−1. bLongest absorption maxima are only reported. cEmission maximum obtained at excitation maximum. dQuantum yields (QY) are measured relative to rhodamine 6G in ethanol (0.92). For the measurements in n-hexane and for 3, absolute QY were measured using an integrating sphere. NE: very weakly emissive. a

dimethyl sulfoxide (DMSO) (Table 1), signified the profound influence of the core, substituents, and the polarity of solvents on absorption and emission spectra. All of the molecules exhibited absorption maximum in the visible region. In the thiazole−het family, a thiophene ring bearing NO2 acceptor at C5 as in 1a−v, contributed to a bathochromic shift in absorption wavelength from 432−470 nm in hexane and from 472−504 in DMSO (refer Table S2 in the SI). However, on placing less aromatic furan ring in the core, the absorption spectra blue-shifted as evident from the representative examples 2a−e, where absorption varied from 424−431 nm and from 459−483 nm in hexane and DMSO, respectively. This observation was rather expected57 since furan with a weaker electronic coupling compared to thiophene would contribute to hypsochromic optical absorbance shift. Thus, along with the computational results predicting variations in the band gap, absorption wavelength and harmonic oscillator strength in various biheterocyclic cores (Figure 2), we now gathered significant experimental results to substantiate the choice of heterocycle unit in the thiazole−het

crystals of a few compounds by slow evaporation in suitable solvents (details in the SI) and recorded the single crystal XRD pattern. Most of the compounds preferred to crystallize in the monoclinic P21/n space group. We further conducted a more detailed investigation on the conformer preferences in different biheteroaryl cores with changing substitution patterns and correlated it with computational results and the findings will appear elsewhere. Structure−Photophysical Property Relationship. With the synthesized molecules in hand, we next evaluated the photophysical properties to illuminate the structure−photophysical property relationship (SPPR) which would facilitate rational design of novel molecules with tunable properties. In a previous report, using computational tools, we discussed the photophysical property tuning in thienylthiazole core.56 Here we are experimentally validating those findings with an expanded library of molecules to demonstrate their potential as fluorophores. The solution-state spectra recorded in six different solvents, viz. n-hexane, toluene, dichloromethane (DCM), tetrahydrofuran (THF), acetonitrile (ACN), and 3457


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The Journal of Organic Chemistry core as the first option for tuning the photophysical properties. Next, we proceeded with analyzing the influence of thiazole ring substituents on the absorption spectra. In the first step, among C2 and C4 positions, the latter was secured by phenyl substitution and former was varied using secondary amines viz; dimethylamine, diethylamines, piperidine, morpholine, and diphenylamine with pKa values of 10.72, 10.98, 11.22, 8.36, and 0.79. With varying donor strength of amines, absorption wavelength was also found to vary and a 13 and 20 nm shift in wavelength was observed when measurements were carried out in nonpolar n-hexane and polar DMSO respectively. Morpholine substitution at C2 produced the highest energy absorptions in nonpolar solvents due to the decreased electron cloud donation ability of morpholine. Similarly, we studied the substituent effect at C4 on the absorption wavelength employing derivatives bearing diphenylamine at C2. As evident from the table, the presence of electron donating groups at C4 red-shifted the absorption wavelength whereas substituents that brought steric crowding contributed to a blue shift in the absorption wavelength in all of the studied compounds. Yet another significant and unique finding from computational study was the prospect of engineering a second chargetransfer channel in the designed systems which was next illustrated by 1u (Figure 5). Here, electron-donating fragments

charge-transfer state whereas the less polarized furanyl systems produced a much clear structured emission. More evidence for the strong ICT behavior which in turn correlated to the large difference in dipole moments between ground and excited states was the pronounced positive solvatochromism exhibited by all of the molecules (Table 1). For example, in the case of 1b, upon increasing the solvent polarity from n-hexane to DMSO, the emission band redshifted by 121 nm (531 nm in n-hexane and 652 nm in DMSO, Figure 6), accounting for a polar excited state which was better

Figure 6. Solvatochromism of 1b demonstrated by emission spectra in various solvents.

stabilized by polar solvents. Further analysis of solvatochromism using a Lippert−Mataga plot gave a good linear relationship (adjacent r2 = 0.97). Substituting the Onsager radius as 5.48 Å (calculated using B3LYP/6-31g(d,p) level theory) for solving the Lippert−Mataga equation, the dipole moment of 1b was found to be 8.47D. The large difference between ground- and excited-state dipole moments revealed the strong ICT in the excited state (more details in the SI). Being built on a multivariant core, the substituent effects on the emission properties would provide significant insights to assess and manipulate the tunability in these fluorophores. Hence, as in the case of absorption studies, with variations at C4 and C2, made one at a time, emissions were measured. For 1a, 1d, 1g, 1i, and 1m, where C4 was phenyl substituted and C2 varied, a 30 nm red shift was observed in DMSO, whereas in nonpolar solvent the shift was minimal (10 nm) (Figure 7a). In nonpolar solvents, morpholine produced the lowest (532 nm in 1i) and piperidine substitution produced the highest emission wavelengths (543 nm in 1g), whereas in polar solvents, piperidine afforded the least (644 nm in 1g) and diphenyl substitution brought the maximum (674 nm in 1m) in emission wavelengths. Similarly, we studied the effect of varying substituents at C4 on the emission behavior by customizing diphenyl as C2 substituent (Figure 7b). It was interesting to note that C4 variation had a considerable effect on emission tunability, demonstrated by 81 nm shift (517 nm for 1v and 598 nm for 1u) in n-hexane. Analogous to the observations in absorption behavior, synergetic effect of electron donations from C2 and C4 of thiazole was noticed here also, accounting for a bathochromic shift in the emission wavelength from a multidirectional charge transfer system. On comparing 1m and 1u, 64 and 96 nm, red-shifted emissions were observed in nhexane and toluene, respectively. The impact of electron rich heterocycle at C4 also was prominent here, producing a red

Figure 5. HOMO and LUMO of 1u calculated at the PBE0/631G(d,p) level of theory to elucidate multidirectional ICT.

diphenylamine and dimethyl aniline placed at C2 and C4, respectively, had a synergistic effect on the absorption wavelength. Further computational calculation of frontier molecular orbitals (FMO) confirmed a larger contribution from the dimethyl aniline fragment to HOMO of 1u and subsequently a second charge transfer channel emanating from C4 to C5 rather than from the usual C2 to C5. Next, we investigated the emission properties of a 30member library comprising of thiazole−het core and πextended systems to validate their fluorophoric potential. Derivatives based on the thiazole−thiophene core with NO2 acceptor on thiophene C5 were highly emissive in nonpolar solvents (QY up to 87%), whereas emission nearly quenched and broadened with increase in solvent polarity as expected for most of the push pull molecules.58,59 In n-hexane, thiazole− furanyl compounds exhibited a structured emission with distinct peaks, whereas in thiazole−thiophene systems a shoulder was observed (see the SI, Figure S2). On increasing polarity, these locally excited emissions were converted to ICT radiations with the disappearance of fine structures and accompanied by a bathochromic wavelength shift. From computational calculations, it was evident that thiophene bearing core with a NO2 substitution was more polar in the ground state than its furan counterpart (from dipole moment values), and hence, in nonpolar hexane they tend to exhibit a 3458


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Figure 7. Effect of substituents on C2 and C4 on the emission wavelengths.

Figure 8. Emission color tunability in toluene (a) and THF (b).

shift in emission in n-hexane (15 nm) and toluene (16 nm) as illustrated for 1p with thiophene as the substituent. Steric bulkiness and increasing conjugation along C4 also had a noticeable effect on the emissions as evident from the comparison of spectra of 1s, 1v, and 1r, where thiophene was the heterocycle in the thiazole−het core construction. In nhexane and DCM, 1s bearing a naphthyl unit fluoresced at 532 and 642 nm, 1v with a diphenyl methyl substituent emitted at 517 and 644, whereas 1r having biphenyl substitution emitted at 542 and 669 nm, which further opened new possibilities and enhanced scope for the modulation of fluorescence wavelength. By perceiving the synthetically realized multi-heterocyclic system where 1,3-thiazole uses its C5 to link to a het unit which in turn can bear a variety of substituents at C5, we were encouraged to analyze the role of C5 substituents on both units in modulating the photophysical properties. To our delight, we found that C5 played the key role in color tunability of the thiazole−het core, which is now discussed with the thiazole− thiophene core. As discussed in the synthesis part, the successful synthesis of 5-(thiazol-5-yl)thiophene-2-carbaldehyde 3 provided the highly versatile aldehyde handle onto C5 of thiophene. Further condensations with malononitrile and 1,3-diethyl-2-thiobarbituric acid afforded 4 and 5, respectively,

resulting in a dramatic shift in photophysical properties. For instance, in THF, 3, 4, and 5 absorb at 409, 496, and 552 nm and emit at 521, 620, and 657 nm, leading to large Stokes shifts of 5256, 4032, 2895 cm−1, respectively. Thus, chemical modification of C5 was identified to be crucial in modulating the π conjugation network and consequently for manipulating the push pull effect. Hence, we propose the molecular engineering of C5 site as a valuable and synthetically viable strategy to afford custom-made fluorophores with a wide range of color tunability (Figure 8). The large Stokes shift (SS) (vide supra) in our systems was highly intriguing since small organic fluorophores with very large SSs are limited in number,60,61 whereas they in high demand62 due to their reduced self-quenching effects sought for use in imaging application. Thus, to assess the potential in imaging applications, we next analyzed the library members for their SS values in varying solvent environments (Figure 9). Interestingly, all of the synthesized small molecules exhibited large SS which increased with solvent polarity as demonstrated for 1d, 1g, 1i, and 1m where the values were 3800, 4056, 4298, and 4052 cm−1, respectively, in n-hexane. A tremendous increase in SS values followed an increase of solvent polarity as demonstrated for the same set of compounds in DMSO with 3459


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compounds. In polar solvents, similar to many of the charge transfer compounds, QY value decreased63,64 and this inverse relationship would help in designing environmental sensitive fluorophores.51 Diphenyl substitution at C2 generally produced a higher QY value as in 1m−v (SI, refer Table S2 also). It was interesting to observe a remarkable effect on QY when a naphthyl unit occupied the C4 position of thiazole. Comparing 1s with 1m, QY decreased in toluene, but there were 11-fold, 2-fold, and 4fold increases which occurred in DCM, THF, and DMSO, respectively. Aldehyde substitution at C5 of the heterocycle brought down the QY in nonpolar solvent as noticed in the case of 3, but it was exciting to note a 33-fold and 59-fold improvement in QY in polar solvents such as ACN and DMSO for 3 as compared to 1m. Long wavelength absorbing molecules 4 and 5 produced an improvement in QY in polar solvents. Another interesting observation was the significant reduction in QY brought about by a shift from the thiazolethiophene core to thiazole−furan core. For example, in toluene, there was a 22, 12, 25, 16, and 13 times decrease in QY for 2a− e compared to corresponding thiophene counterparts. However, effect of substituent on QY variation was not fully understood and further analysis with an expanded library has to be performed for a logical conclusion. To summarize the key findings from the experimental SPPR studies: (i) both the core and the substituents along charge transfer directions are crucial in deciding the photophysical properties, (ii) C5 position is crucial in full color tunability (demonstrated in Figure 11), (iii) presence of electron donating groups at C4 of thiazole induce red shift whereas bulky groups bring blue-shifted absorptions and emissions, (iv) donor ability of amine groups at C2 influences absorption, and (v) photophysical properties can be tuned according to solvent polarities. Computational Calculations. With the wealth of information from structure−activity studies and the interesting photophysical properties resulting from charge transfer, we next proceeded with DFT and TD-DFT calculations to gain deeper insights on the observed phenomena. The geometries were optimized using PBE0/6-31g(d,p) level of theory and the subsequently performed vibrational frequency calculations identified the minimum in the potential energy surfaces. Inspection of electronic densities in the Kohn−Sham FMOs confirmed the charge-transfer nature of the molecule upon electronic excitations (Figure 12). In the HOMO, electron density was evenly distributed over the entire molecules,

Figure 9. Variation of Stokes shift in different solvents.

values 5243, 4593, 5460, and 5867 cm−1, respectively. In nonpolar solvents, morpholine at C2 produced the largest SS (4385 cm−1) whereas in polar solvents diphenyl group afforded largest values (more than 6500 cm−1). However, a change of C5 substituent in thiophene core or shifting to pi extended systems brought down the SS shift considerably. Quantum yield (QY), being an important parameter, was next determined in various solvents to assess the fluorophores. QY of 1a−v in various solvents revealed that they were highly emissive in toluene and moderately in n-hexane as well as in THF. Figure 10 depicts the QY variation among selected

Figure 10. Plot to indicate the variation in QY of selected compounds.

Figure 11. Overall emission tunability obtained for the synthesized molecules in different solvents. 3460


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Figure 12. Kohn−Sham frontier molecular orbitals of selected compounds calculated using PBE0/6-31G(d,p).

Figure 13. PDOS calculation: Percentage contribution of each fragment to HOMO and LUMO.

Further, TD-DFT calculations were performed to correlate the observed photophysical properties of the synthesized molecules for which selected molecules were optimized in THF using PBE0/6-31g(d,p) theory, and vertical excitation energies for the lowest excited states were calculated using single point calculations. Polarizable continuum model (PCM) was used to account for the solvent effects. PBE0 successfully reproduced the trend in experimentally observed absorption wavelengths and are listed in the Table 2. The HOMO−

whereas in LUMO, the amine and thiazole fragments were deprived of their electrons which were seen localized more on the acceptor fragment. Moving from thiazole-thiophene core to thiazole−furan core, electron density in LUMO of the latter as in 2a was a little higher in aminothiazole and lesser in acceptor fragment which increased the band gap in these molecules. The variations in properties brought in by changes in the fragments used for the construction of these multi-heterocyclic systems further encouraged us to gain a deeper understanding of the percentage contribution of each fragment to the HOMO and LUMO. We performed partial density of states (PDOS) calculations and the results were analyzed with the help of GaussSum 3.0 software.65 For the calculations, each molecule was divided into five segments comprising amino donor (D), thiazole (T), fragments attached to thiazole at C4 (C4), heterocycles attached to thiazole at C5 (Het), and the acceptor group on C5 of Het (A). The percentage contribution toward HOMO and LUMO from each fragment is depicted in the Figure 13. In thiazole−thiophene systems, a well-defined charge separated state was formed when compared to their furan equivalents. The D−T part contributed 77 and 70% to HOMO, and the Het−A part contributed 84 and 73% to LUMO in 1m and 2e, respectively, with noteworthy influence on the photophysics. On moving from HOMO to LUMO, a 21, 36, and 43% difference in density was observed in 3, 4, and 5, respectively. Thus, it was concluded that a stronger acceptor has a greater contribution in LUMO which favored much stronger electronic coupling and increased the ICT character and hence shift in absorption and emission maxima. The second charge transfer channel was also evident from the PDOS calculations demonstrated by comparing 1m and 1u. It was observed that D and C4 segment contribute 37 and 8% to HOMO in 1m, whereas in 1u it was 9 and 54%, respectively.

Table 2. TD-DFT Calculation Results of Selected Solvents in THF PBE0/6-31G(d,p) code

λexp (nm) in THF

λTD‑DFT (nm)

eV

f

1m

460

493

2.51

0.4941

2a

455

444

2.79

0.6426

3

409

438

2.83

0.5480

4

496

531

2.34

0.8146

5

552

558

2.22

0.9638

major contributions HOMO→LUMO (99%) HOMO→LUMO (100%) HOMO→LUMO (99%) HOMO→LUMO (99%) HOMO→LUMO (99%)

LUMO transition was found to be the major one responsible for the longest wavelength absorption band. These results are highly encouraging and will help in the rational design of molecules with tunable wavelengths within the realm of synthetic flexibility. In Vitro Anticancer Screening. Interesting photophysical properties and their tunability encouraged us to explore the therapeutic potential of these systems for envisaged applications as single molecule theranostics. We performed prelimi3461


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study on computational evaluation of optoelectronic properties of the library is in progress. (b). Evaluation of Sensing Properties. Since modulation of ICT character in D−A compounds has a significant impact on their functional properties and amine donor groups are plausible sites for protonation,69 we felt it worthy to investigate the sensor properties. To gather first-hand information, we relied on computational calculations performed on the protonated form. The map of electrostatic potential of 1g and its protonated form is shown Figure 14, and we noticed

nary in vitro screening of two representative members, 1a and 1g, for their anticancer activity in six human cancer cell lines, MCF-7 (breast cancer), A-549 (lung cancer), HL-60 (leukemia), OVCAR-3 (ovarian cancer), SK-MEL-2 (melanoma), and SW-620 (colon cancer), using SRB assay with adriamycin as the positive control, and the results are shown in Table 3. Promising results obtained in HL-60 (GI50 8.0 μg/ Table 3. In Vitro Screening Results of Representative Molecules in Six Cancer Cell Lines GI50a

a

cell lines

1a

1g

adriamycin

MCF-7 HL-60 OVCAR-3 SK-MEL-2 SW-620 A-549

44.0 8.0 >100 >100 56.7 56.7

34.3 NE >100 >100 43.4 >100

Computational Design, Synthesis, and Structure Property Evaluation

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